Gasoline, which is a fuel for automobiles, is highly volatile. Thus, driving and parking under the scorching sun cause vaporization of gasoline in a fuel tank, resulting in gasoline vapor emission into the air. Also, gasoline vapor is generated during fueling.
Therefore, in order to prevent gasoline vapor emission to the outside of vehicles, a canister is provided to vehicles such that activated carbon, which is an adsorbent contained in such a canister, adsorbs gasoline vapor. For example, a canister capable of adsorbing/desorbing fuel vapor is provided to an automobile internal combustion engine in order to prevent fuel vapor emission to the outside of a vehicle, such vapor evaporating from a fuel tank. In this case, a canister temporarily adsorbs fuel vapor generated after, for example, stopping of a vehicle and desorbs the adsorbed fuel components with fresh air during the subsequent driving for combustion treatment in an internal combustion engine.
In addition, an adsorbent in a canister adsorbs gasoline vapor generated upon gasoline fueling. Adsorbed gasoline vapor is desorbed (purged) from activated carbon as a result of engine rotation. Then, the vapor is mixed with intake air from the outside, guided through an intake air pipe to an engine, and burned.
In general, a granular activated carbon or a fibrous activated carbon is used as an adsorbent provided to an intake air system. In the case of a recently devised technique, a decrease in the adsorption capacity of an adsorbent can be prevented by limiting the pore distribution of a fibrous activated carbon so as to cause excellent desorption of an adsorbate. However, even with the use of such a fibrous activated carbon, the adsorption capacity decreases with long-term use in some cases. In addition, when such activated carbon is used while immobilized on a filter or the like, detachment of the activated carbon occurs and the airflow resistance of filter (such airflow being directed toward the inside of an internal combustion engine) increases, which have been problematic.
Therefore, in order to obtain a fuel vapor adsorbent that is unlikely to deteriorate even after long-term use, JP Patent Publication (Kokai) No. 2003-314387 A discloses a granular adsorbent having pores with pore sizes of 1.4 nm to 2.8 nm (50% or more pores) and a pore volume of 0.3 ml or more per 1 ml of adsorbent, which is provided inside an intake air system of an internal combustion engine.
As an aside, the form of a conventional activated carbon can be selected from a powder form, a granular form, a shaped form (e.g., a cylindrical form, a block form, a briquette form, or a spherical form). However, in a case in which an activated carbon with a large capacity per unit volume is required, forced shaping is carried out in many cases in order to increase the particle density. In such case, it is difficult to obtain an activated carbon in a spherical form. That is, a general spherically shaped product has a low particle density as a result of self-granulation, such as rolling granulation. Thus, when such product is subjected to advanced activation so as to be used for automobiles and the like, the capacity per unit volume does not increase, resulting in a decrease in hardness.
For example, activated carbon with the large adsorption rate per unit volume is required for automobile canisters. However, even when a container is filled with such activated carbon to a maximum extent, there arises a problem regarding ventilation pressure loss. Even in the case of an activated carbon in a spherical form that is appropriate for packing, the adsorption capacity per unit volume is not exhibited if the particle density is low, which results in a decrease in the working capacity (WC: adsorption/desorption capacity). Further, activated carbon with low particle density has a low degree of hardness, resulting in cracking or powdering. This is problematic for use.